Housings for weighing apparatuses are known in diverse forms. Known housings for weighing apparatuses frequently include a bottom plate, on which the weighing apparatus is located. The weighing apparatus can, for example, comprise a sensor, which works in accordance with the principle of the electromagnetic force compensation. Such force sensors usually have a Roberval mechanism, which is, if necessary, connected to one or more transmission levers. The load force to be detected acts on the movable part of the Roberval mechanism, which is connected to the first transmission lever. There is a coil at the end of the last lever; it is held at a constant height position in a magnetic field. To this end, the coil located on the last lever is impinged on with a current, which is regulated in such a way that with the currently acting load force, the predetermined height position of the last lever is maintained constant. Thus, the value of the current through the coil is a measure of the detected load force. All the components that are required for the detection of the load force, that is, the sensor, the control electronics, and perhaps a device for the processing of the detected measurement value(s), can be situated on the bottom plate of the housing. The housing usually includes a housing upper part which covers the upper side of the bottom plate for protection against environmental influences, such as, in particular, dust and moisture.
A carrier arm, on the one hand, and the contacts to emit the electrical or electronic measurement signals, on the other hand, must, however, be conducted out of the housing. The carrier arm is usually conducted out via an opening on the upper side or cover side of the housing. The opening is also sealed, with respect to environmental influences, by means of a sealing arrangement. In particular, labyrinth seals can be considered, wherein a first sealing part in the opening of the housing, or surrounding the opening, is located on the upper side of the housing. This first sealing part is penetrated by the carrier arm, if possible, in a non-contact manner. In this case, the carrier arm is connected to a second sealing part, which, together with the first sealing part, forms a labyrinth-like annular space in a vertical section. In this way, a contact-free sealing of the opening, without thereby resulting in a force shunt, is attained in the housing for the carrier arm.
Cable conduits or plug contacts for the transmission of the electrical measurement signals from the housing can be provided on the underside of the housing, that is, on the underside of the bottom plate.
Such units, including a housing and a weighing apparatus situated therein, are designated as weighing cells within the scope of this description.
Such weighing cells are frequently incorporated into complex weighing devices, for example, multitrack weighing apparatuses. To this end, the weighing cells are usually installed with their bottom plate on a machine frame of the complex weighing device.
For sealing weighing apparatus housings that include a bottom plate and a pot- or tub-shaped housing upper part, various methods are known. It is possible, for example, to weld, to cement, or to seal off tightly the two housing parts, as hermetically as possible, by sealing elements arranged between the housing parts. The sealing elements may include flat seals or round cord seals (O-rings) arranged between the housing upper part and the upper side of the base plate.
Such flat seals or round cord seals are acted on with a defined force during the installation of the weighing cell, that is, during the joining of the two housing parts, and are thereby deformed. Due to this deformation and due to the aging processes of the materials of the seals or due to wear and tear, for example, produced by contact with aggressive media, the sealing elements will frequently eventually lose their sealing effect. The actual condition of the sealing element, and thus of the sealing effect, can be seen only with difficulty when the housing parts have been installed, since the sealing elements are usually completely situated between the housing parts to be sealed. For example, the housing upper part with its side walls can encompass the outside of the bottom plate, and the sealing element can be provided between the inside of the side walls and the outside of the bottom plate. For sealing elements to engage laterally, the side walls of the housing upper part can be connected to the bottom plate, using laterally provided, horizontal screw connections. Alternatively, the side walls of the housing upper part in the lower part can also be bent laterally outward or inward and screwed to the bottom plate, from above or below, wherein the sealing element is provided between the bent wall part and the upper side of the bottom plate. Since the sealing element in these cases is not visible from the outside and thus its condition cannot be examined without dismantling the housing, a defective sealing effect is frequently not recognized and is determined only with the occurrence of functional damage to the weighing cell.
In order to avoid such damage to the weighing cell, it is possible, for example, to retighten the installation screws at certain predetermined chronological intervals, so as to again produce the original pressing force on the sealing element. Alternatively, of course, the sealing element can also be replaced at predetermined chronological intervals. To this end, however, an at least partial dismantling of the weighing cell is necessary, which should usually take place only at the manufacturing factory, so as to guarantee the required accuracy and reliability of the weighing cell.
For the inspection of the condition of a force measuring apparatus, EP 2 130 007 B1 describes a method in which the interior of the weighing cell is filled with a gas. To detect leakages, the state of the gas filling within the weighing cell housing is monitored. However, this is technically cumbersome and expensive.
In addition, in many cases, there is the problem of sealing the housing of the weighing cell with respect to a carrier element, for example, a machine frame. This is always sensible and necessary if the weighing cell has cable outlets through the bottom plate downward or plug contacts that lead out on the underside of the bottom plate for the conducting away of measurement signals or the delivery of control signals.